1. Field of the Invention
The present invention relates to a fading equalizer, and in particular to a fading equalizer that is employed in a demodulator of a radio receiver for performing communication in a Rayleigh (uniform) fading environment, such as vehicular communication.
2. Related Arts
Recently, in consonance with the wide use of vehicular communication, high quality transmission, such as data communication, having a small error rate is demanded. Communication performed while moving, however, is affected by uniform fading according to the Doppler shift. Therefore, even for communication performed at a high average reception signal level (S/N), line or channel quality is not improved and data transmission efficiency is not enhanced.
Assume that a series of symbols that represent quadrature detect output are I(n) and Q(n) (n: natural number); a clock having a natural number, which is an integral times a symbol rate R.sub.s that is synchronous with the symbol sequence, is S.sub.CLK ; and a presumed maximum fading frequency, so called a Doppler frequency is F.sub.max.
A conventional demodulator adopts a delay detect method that is comparatively less affected by uniform fading. In addition, there is another method whereby a datum signal, which is a known signal is inserted at intervals of equal to or smaller than 1/(2F.sub.max) on the transmission side, and a reception side utilizes the datum signal to compensate for fading.
However, in the delay detecting method, even through the average reception signal level is high, the line quality, that is a bit error rate is not improved, and this is so called as error rate floor phenomenon.
In the fading compensation method whereby a datum signal is inserted, although the floor phenomenon for an error rate can be avoided, data transfer efficiency is reduced because of the insertion of a datum signal.